As an image forming apparatus, a color image forming apparatus is known in which images of respective colors formed on photoreceptors in plural image formation stations are superimposed on a record medium or a transfer belt. In the image forming apparatus as stated above, it is necessary that plural images formed in the plural image formation stations are accurately superimposed on the transfer belt.
Thus, hitherto, in each of the plural image formation stations, an adjustment pattern formed on the transfer belt is detected, and an adjustment value obtained based on the detection result is used to correct an image shift. On the other hand, as an image forming apparatus, there is a color image forming apparatus in which plural process speeds are changed and image formation is performed. In the color image forming apparatus in which the plural process speeds are changed, hitherto, it is necessary that an adjustment value is obtained each time the process speed varies and an image shift is corrected.
However, when the adjustment value is obtained each time the process speed varies and the image shift is corrected, each time the process speed is changed, it takes labor to perform the image shift correction, and it takes time to correct the image shift. Thus, it takes time to shift to another process speed, and there is a fear that improvement in productivity is hindered.
Then, in an image forming apparatus having plural process speeds, it is desirable to develop an image shift adjusting apparatus of the image forming apparatus, which shortens a time required for image shift correction when the process speed is changed and can improve the productivity of images.